This invention relates generally to optical communications and, particularly, to arrayed waveguide gratings. Arrayed waveguide gratings are also sometimes called waveguide grating routers or phased array waveguide gratings.
In wavelength division multiplexed optical signals, a plurality of different optical signals, each having a different wavelength, may be multiplexed over the same optical length. At intended destinations, one or more of the wavelength signals may be separated using a demultiplexing technique.
An arrayed waveguide grating works like a diffraction grating. It may be fabricated as a planar structure including input and output waveguides, input and output waveguide couplers, and arrayed waveguides. The length of any arrayed waveguide may differ from adjacent waveguides by constant ΔL.
The input waveguide coupler splits the wavelength channels equally among the arrayed waveguides. Each portion of the input light traveling through the arrayed waveguide includes all the wavelengths that have entered the grating. Each wavelength in turn is individually phase shifted. As a result of that phase shift and phase shifts at the input/output waveguide couplers, every portion of the light at a given wavelength sees different phase shifts. These portions may interfere at the output waveguide coupler, producing a set of maximum light intensities. The direction of each maximum intensity depends on its wavelength. Thus, each wavelength is directed to an individual output waveguide.
Arrayed waveguide gratings should ideally have low insertion loss, low crosstalk, broad bandwidth, and a flat pass band. However, conventional arrayed waveguide gratings exhibit aberrations in the transmission spectra. Specifically, when the arrayed waveguide gratings are designed according to the standard Rowland mounting, the transmission spectra are symmetric only for the center channels and are increasingly asymmetric for the edge channels.
Conventionally it is believed that these aberrations are phase aberrations. Thus, efforts to correct these aberrations have focused on correcting the phase aberration.
Thus, there is a need for better ways to correct the aberration in arrayed waveguide gratings.